A 17-year-old boy with hemophilia A and strong inhibitors for factor VIII suffered a myocardial infarction on the sixth day of treatment with large amounts of human factor IX complex (Konyne). The patient also had had diabetes for one year, for which he was receiving insulin injections, but there was no evidence of vascular disease. Thromboembolic complications that occur after the administration of factor IX concentrates have been reported in patients with hemophilia B or liver disease, but these complications are extremely rare in patients with classic hemophilia. (JAMA 1981;245:1455-1456)